The majority of commercial organic liquids are available on the market in a purity which is already very high, generally higher than 99%. However, as the data in Table 1 show, metals in trace form are still found in these liquids, which require an additional purification to permit their use in industries such as electronics or pharmacy. In general, organic liquids containing less than 10 ppb of each alkali and alkaline-earth metal and metal contaminant would be necessary for most of the uses in these two technical fields (1 ppb=1 part by weight per thousand million, that is 1 .mu.g per kg).
TABLE 1 ______________________________________ Compound 1-methyl- isopropyl N,N- mono- benzyl 2-pyrrol- alcohol dimethyl- ethanol- alcohol idone acetamide amine Supplier S.G.S. S.D.S. Aldrich Huls Elf Atochem Grade ultra- purex for H.P.L.C. ultra- photo- pure analysis pure graphic grade Purity (%) 99.95 99.7 99.9 99.90 99.90 Water (%) 0.02 0.1 &lt;0.03 0.06 &lt;0.1 ______________________________________ Detec- tion limit Metal (ppb) Content as metal (ppb) ______________________________________ Na 2 30 30 150 20 240 Fe 1 15 6 85 5 5 ______________________________________
It therefore appears desirable to have available a process for the purification of commercial liquids containing one or more organic compounds, which are already of a good purity, but nevertheless insufficient for some applications, this process being aimed especially at reducing their content of traces of metals.
Ion exchange resins are very commonly employed nowadays for deionizing water. On the other hand, their use in an organic medium appears to be markedly less widespread or investigated. This absence of development owes its origins to the special properties of water, which ionizes salts and separates the anions completely from the cations. On the other hand, depending on the dielectric constant of the organic medium, the ions formed by ionization are found to be more or less dissociated and more or less free to exchange with the functional groups of the resin.
Nevertheless some published research is found (C. A. Fleming and A. J. Monhemus, Hydrometallurgy, 4, pp. 159-167, 1979), the objective of which is to improve by a solvent effect the selectivity of exchange of some metals with cationic resins, the final aim being to determine the conditions which permit the separation of the metals in preparative ion chromatography. These studies describe exchange isotherms, that is to say the laws which govern the equilibrium between the metal ion in solution and the metal ion bound to the resin. The usual conditions of such work consequently remain very far from a method of deionization of an organic medium.
U.S. Pat. No. 4,795,565 describes the purification of an aqueous solution of ethanolamine on an ion exchange resin. The objective of this patent is the removal of some salts produced during the ethanolamine extraction of carbon dioxide and hydrogen sulphide present in refinery gases. The spent ethanolamine solution containing between 80 and 50% water by weight is passed successively over a stationary bed of strong anionic resin and then over a stationary bed of strong cationic resin. U.S. Pat. No. 5,162,084 relates to the same type of application, but improves the purification efficiency by employing a combination of two anionic resins and by judiciously controlling the operation of the unit with the aid of conductimetric sensors. These patents do not describe the purification of ethanolamine with a low water content on ion exchange resins.
Patent GB 2 088 850 describes the purification of 1-methyl-2-pyrrolidone (NMP) by passing over an anionic resin binding the chloride and carboxylic ions. This treatment is inserted into a process of selective extraction with NMP of the aromatic hydrocarbons present in a mixture of paraffins. No purification with cationic resins is described in this patent. Furthermore, 10% of water by weight is advantageously added to the NMP to improve the selectivity of extraction.
Patent RU 2 032 655 relates to the deionization of aliphatic alcohols or of diols with the aim of reducing their electric conductivity. For this purpose the authors employ a stationary bed of anionic resin and of cationic resin in equal proportions, these resins being saturated with water beforehand. In a subsequent publication (Vysokochist. Veshchestva, 2, pp. 71-75, 1992), A. G. Myakon'kii et al. have indicated that a minimum water content of 2.5% in the medium is necessary to obtain deionization with the aid of the pair of dry resins.
In an article intitled "Novel resin-based ultrapurification system for reprocessing IPA in the semiconductor industry" (Ind. Eng. Chem. Res. 1996, 35, 3149-3154), P. V. Buragohain et al. propose using cation exchange resins (Amberlite.RTM. IR 120, Dowex.RTM. M31 and Ionac.RTM. CFP 110) to purify isopropylic alcohol (IPA). In these cationic resins, the divinylbenzene content in the copolymer does not exceed 20%.
The use of an ion exchange resin of sulphonic type which has its active groups in SO.sub.3 H acidic form for the purification of dimethyl sulphoxide (DMSO) forms the subject matter of the patent application WO97/19057 published on May 29, 1997.